Inhibition of Pi4kb activity causes malformation of vestibular apparatus in zebrafish by downregulating hey1

Phosphatidylinositol 4 kinase-β (PI4KB) plays critical roles in human genetic diseases. In zebrafish, Pi4kb is strongly expressed in hair cells (HCs), which are necessary for detecting sound vibrations, head movements, and water motion. However, the role of PI4KB in HC or semicircular canal development is unclear. Herein, we report that pi4kb morphants exhibit insensitivity to sound stimulation and abnormal morphological vestibular organs, including cilium loss in HCs of the cristae and semicircular canal malformation. As bone morphogenetic protein (BMP) signaling is associated with HC and semicircular canal development, we analyzed the expression of BMP-related genes; the phosphorylated Smad1/5/9 (p-Smad1/5/9) expression was markedly reduced in otic HCs. RNA-sequencing data indicated that the transcriptional levels of BMP membrane receptor 2 (bmpr2a and bmpr2b) and hes-related family of bHLH transcription factors with YRPW motif 1 (hey1), a direct downstream target gene of p-Smad, were significantly reduced in the pi4kb morphants, as verified using quantitative reverse transcription-polymerase chain reaction and in situ hybridization. Co-injection of hey1 mRNA and pi4kb morpholino notably recovered vestibular apparatus development, including the number and length of cilia in HCs of the cristae and semicircular canal formation. Collectively, these results suggest that Pi4kb is involved in vestibular apparatus development in zebrafish by regulating BMP membrane receptor 2 and p-Smad1/5/9 levels, thereby affecting the transcriptional activation of the target gene hey1. This study sheds light on the interaction between Pi4kb and the BMP-Hey1 signaling axis, which is critical for HC and semicircular canal formation.

Nestin-expressing cells are mitotically active in the mammalian inner ear

Nestin expression is associated with pluripotency. Growing evidence suggests nestin is involved in hair cell development. The objective of this study was to investigate the morphology and role of nestin-expressing cells residing in the early postnatal murine inner ear. A lineage-tracing nestin reporter mouse line was used to further characterize these cells. Their cochleae and vestibular organs were immunostained and whole-mounted for cell counting. We found Nestin-expressing cells present in low numbers throughout the inner ear. Three morphotypes were observed: bipolar, unipolar, and globular. Mitotic activity was noted in nestin-expressing cells in the cochlea, utricle, saccule, and crista. Nestin-expressing cell characteristics were then observed after hair cell ablation in two mouse models. First, a reporter model demonstrated nestin expression in a significantly higher proportion of hair cells after hair cell ablation than in control cochleae. However, in a lineage tracing nestin reporter mouse, none of the new hair cells which repopulated the organ of Corti after hair cell ablation expressed nestin, nor did the nestin-expressing cells change in morphotype. In conclusion, Nestin-expressing cells were identified in the cochlea and vestibular organs. After hair cell ablation, nestin-expressing cells did not react to the insult. However, a small number of nestin-expressing cells in all inner ear tissues exhibited mitotic activity, supporting progenitor cell potential, though perhaps not involved in hair cell regeneration.

The dark kinase STK32A regulates hair cell planar polarity opposite of EMX2 in the developing mouse inner ear

The vestibular maculae of the inner ear contain sensory receptor hair cells that detect linear acceleration and contribute to equilibrioception to coordinate posture and ambulatory movements. These hair cells are divided between two groups, separated by a line of polarity reversal (LPR), with oppositely oriented planar-polarized stereociliary bundles that detect motion in opposite directions. The transcription factor EMX2 is known to establish this planar polarized organization in mouse by regulating the distribution of the transmembrane receptor GPR156 at hair cell boundaries in one group of cells. However, the genes regulated by EMX2 in this context were previously not known. Using mouse as a model, we have identified the serine threonine kinase STK32A as a downstream effector negatively regulated by EMX2. Stk32a is expressed in hair cells on one side of the LPR in a pattern complementary to Emx2 expression in hair cells on the opposite side. Stk32a is necessary to align the intrinsic polarity of the bundle with the core planar cell polarity (PCP) proteins in EMX2-negative regions, and is sufficient to reorient bundles when ectopically expressed in neighboring EMX2-positive regions. We demonstrate that STK32A reinforces LPR formation by regulating the apical localization of GPR156. These observations support a model in which bundle orientation is determined through separate mechanisms in hair cells on opposite sides of the maculae, with EMX2-mediated repression of Stk32a determining the final position of the LPR.

A comparative study of vestibular improvement and gastrointestinal effect of betahistine and gastrodin in mice

Betahistine and gastrodin are the first-line medications for vestibular disorders in clinical practice, nevertheless, their amelioration effects on vestibular dysfunctions still lack direct comparison and their unexpected extra-vestibular effects remain elusive. Recent clinical studies have indicated that both of them may have effects on the gastrointestinal (GI) tract. Therefore, we purposed to systematically compare both vestibular and GI effects induced by betahistine and gastrodin and tried to elucidate the mechanisms underlying their GI effects. Our results showed that betahistine and gastrodin indeed had similar therapeutic effects on vestibular-associated motor dysfunction induced by unilateral labyrinthectomy. However, betahistine reduced total GI motility with gastric hypomotility and colonic hypermotility, whereas gastrodin did not influence total GI motility with only slight colonic hypermotility. In addition, betahistine, at normal dosages, induced a slight injury of gastric mucosa. These GI effects may be due to the different effects of betahistine and gastrodin on substance P and vasoactive intestinal peptide secretion in stomach and/or colon, and agonistic/anatgonistic effects of betahistine on histamine H1 and H3 receptors expressed in GI mucosal cells and H3 receptors distributed on nerves within the myenteric and submucosal plexuses. Furthermore, treatment of betahistine and gastrodin had potential effects on gut microbiota composition, which could lead to changes in host-microbiota homeostasis in turn. These results demonstrate that gastrodin has a consistent improvement effect on vestibular functions compared with betahistine but less effect on GI functions and gut microbiota, suggesting that gastrodin may be more suitable for vestibular disease patients with GI dysfunction.

Bidirectional Transport of IgE by CD23 in the Inner Ear of Patients with Meniere's Disease

Meniere's disease (MD) is a disorder of the inner ear characterized by episodes of spontaneous vertigo, fluctuating hearing loss, and tinnitus. Recent studies have demonstrated that IgE may play a role in the pathogenesis of MD. Patients with MD (n = 103), acoustic neuroma (n = 5), and healthy subjects (n = 72) were recruited into the study. Serum from the participants was analyzed for IgE and type 2-related cytokines. IgE and CD23 expression levels in vestibular end organs of patients, C57BL/6 mice, or mouse HEI-OC1 cells were analyzed. Finally, the role of CD23 in IgE transcytosis was assessed using HEI-OC1 cells. Serum IgE was elevated in patients with MD and positively correlated with clinical symptoms. IL-4, IL-5, IL-10, IL-13, and CD23 levels were increased in patients with MD compared with the control group. In the transcytosis assay, mouse IgE was found to be bidirectionally transported across the HEI-OC1 cell monolayer. Additionally, CD23 downregulation using a small interfering RNA approach significantly reduced the efficiency of IgE transcytosis, suggesting that IgE is transported by CD23. Furthermore, exposure to IL-4 increased CD23 expression and enhanced IgE transcytosis in the HEI-OC1 cells and primary vestibular end organs. Our study indicated that IgE may play a role in the pathophysiology of MD. In addition, CD23-mediated IgE transcytosis in the hair cells may play a critical role in initiating inflammation in the inner ear. Thus, reducing the level of IgE may be a potentially effective approach for MD treatment.

Effect and mechanism of BDNF/TrkB signaling on vestibular compensation

Brain-derived neurotrophic factor (BDNF) regulates neuronal plasticity by targeting the tyrosine kinase B receptor (TrkB) receptor, but limited researches concentrate on the role of BDNF/TrkB signaling in vestibular compensation. In this study, rats with unilateral vestibular dysfunction were established by unilateral labyrinthectomy (UL) and infusion with siBDNF or 7, 8-Dihydroxyflavone (7,8-DHF, a TrkB receptor agonist). The behavioral scores of rats with vestibular deficits were determined and the rotarod test was performed after UL. BDNF and TrkB levels after UL were determined by western blot and quantitative reverse transcription PCR (qRT-PCR). 5-bromo-2'-deoxyuridine (BrdU)-positive cells (newly generated cells) and GAD67-positive cells (GABAergic neurons) were identified by immunohistochemistry. Glial fibrillary acidic protein (GFAP) (astrocyte marker)-positive cells were identified and GABA type A receptor (GABAAR) expression was detected by immunofluorescence. We found that after UL, BDNF and TrkB levels were up-regulated with a maximum value at 4 h, and then progressively down-regulated during 4 h ~ 7 d. Blocking BDNF/TrkB signaling inhibited the recovery from vestibular deficits, decreased the numbers of newly generated cells and astrocytes in medial vestibular nucleus (MVN), inferior vestibular nerve (IVN), superior vestibular nerve (SVN) and lateral vestibular nucleus (LVN), and disrupted the balances of GABAergic neurons and GABAAR expressions in the left (lesioned) side and right (intact) side of MVN, whereas activation of BDNF/TrkB signaling caused opposite results. The current study indicated that BDNF/TrkB signaling avails vestibular compensation, depending on the number of newly generated cells and astrocytes, the rebalance of GABAergic neurons, and GABAAR expression in bilateral MVN.

Retinoic acid degradation shapes zonal development of vestibular organs and sensitivity to transient linear accelerations

Each vestibular sensory epithelium in the inner ear is divided morphologically and physiologically into two zones, called the striola and extrastriola in otolith organ maculae, and the central and peripheral zones in semicircular canal cristae. We found that formation of striolar/central zones during embryogenesis requires Cytochrome P450 26b1 (Cyp26b1)-mediated degradation of retinoic acid (RA). In Cyp26b1 conditional knockout mice, formation of striolar/central zones is compromised, such that they resemble extrastriolar/peripheral zones in multiple features. Mutants have deficient vestibular evoked potential (VsEP) responses to jerk stimuli, head tremor and deficits in balance beam tests that are consistent with abnormal vestibular input, but normal vestibulo-ocular reflexes and apparently normal motor performance during swimming. Thus, degradation of RA during embryogenesis is required for formation of highly specialized regions of the vestibular sensory epithelia with specific functions in detecting head motions.

Ocular vestibular evoked myogenic potential (VEMP) reveals mesencephalic HTLV-1-associated neurological disease

Purpose

Vestibular Myogenic Evoked Potential (VEMP) evaluates vestibulo-ocular and vestibulo-collic reflexes involved in the function of the otolithic organs and their afferent pathways. We compared the results of cervical and ocular VEMP in HTLV-1 associated myelopathy (HAM) and HTLV-1-asymptomatic infection.

Participants and methods

This cross-sectional study included 52 HTLV-1-infected individuals (26 HAM and 26 asymptomatic carriers) and 26 seronegative controls. The groups were similar regarding age and gender. Participants underwent simultaneous ocular and cervical VEMP. The stimulus to generate VEMP was a low-frequency tone burst sound tone burst, with an intensity of 120 decibels normalized hearing level, bandpass filter from 10 to 1,500 Hertz (Hz), with 100 stimuli at 500 Hz and 50 milliseconds recording time. The latencies of the electrophysiological waves P13 and N23 for cervical VEMP and N10 and P15 waves for ocular VEMP were compared among the groups. The absence or delay of the electrophysiological waves were considered abnormal results.

Results

Ocular VEMP was similar among the groups for N10 (p = 0.375) and different for P15 (p≤0.001). Cervical VEMP was different for P13 (p = 0.001) and N23 (p = 0.003). About ocular VEMP, in the HTLV-1-asymptomatic group, normal waves were found in 23(88.5%) individuals; in HAM group, normal waves were found in 7(26.9%). About cervical VEMP, 18(69.2%) asymptomatic carriers presented normal waves and only 3(11.5%) patients with HAM presented normal waves. Abnormalities in both VEMPs were found in 1(3.8%) asymptomatic carrier and in 16(61.5%) patients with HAM.

Conclusion

Neurological impairment in HAM was not restricted to the spinal cord. The mesencephalic connections, tested by ocular VEMP, have been also altered. Damage of the oculomotor system, responsible for eye stabilization during head and body movements, may explain why dizziness is such a frequent complaint in HAM.

Effects of hypergravity on the gene expression of the hypothalamic feeding-related neuropeptides in mice via vestibular inputs

The effects of hypergravity on the gene expression of the hypothalamic feeding-related neuropeptides in sham-operated (Sham) and vestibular-lesioned (VL) mice were examined by in situ hybridization histochemistry. Corticotrophin-releasing hormone (CRH) in the paraventricular nucleus was increased significantly in Sham but not in VL mice after 3 days of exposure to a 2 g environment compared with a 1 g environment. Significant decreases in pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript and significant increases in neuropeptide Y, agouti-related protein in the arcuate nucleus and orexin in the lateral hypothalamic area were observed in both Sham and VL mice. After 2 weeks of exposure, CRH and POMC were increased significantly in Sham but not in VL mice. After 8 weeks of exposure, the hypothalamic feeding-related neuropeptides were comparable between Sham and VL mice. These results suggest that the hypothalamic feeding-related neuropeptides may be affected during the exposed duration of hypergravity via vestibular inputs.

Differential regulation of NMDA receptor-expressing neurons in the rat hippocampus and striatum following bilateral vestibular loss demonstrated using flow cytometry

There is substantial evidence that loss of vestibular function impairs spatial learning and memory related to hippocampal (HPC) function, as well as increasing evidence that striatal (Str) plasticity is also implicated. Since the N-methyl-d-aspartate (NMDA) subtype of glutamate receptor is considered essential to spatial memory, previous studies have investigated whether the expression of HPC NMDA receptors changes following vestibular loss; however, the results have been contradictory. Here we used a novel flow cytometric method to quantify the number of neurons expressing NMDA receptors in the HPC and Str following bilateral vestibular loss (BVL) in rats. At 7 and 30 days post-op., there was a significant increase in the number of HPC neurons expressing NMDA receptors in the BVL animals, compared to sham controls (P ≤ 0.004 and P ≤ 0.0001, respectively). By contrast, in the Str, at 7 days there was a significant reduction in the number of neurons expressing NMDA receptors in the BVL group (P ≤ 0.05); however, this difference had disappeared by 30 days post-op. These results suggest that BVL causes differential changes in the number of neurons expressing NMDA receptors in the HPC and Str, which may be related to its long-term impairment of spatial memory.

Three-dimensional Organotypic Cultures of Vestibular and Auditory Sensory Organs

The sensory organs of the inner ear are challenging to study in mammals due to their inaccessibility to experimental manipulation and optical observation. Moreover, although existing culture techniques allow biochemical perturbations, these methods do not provide a means to study the effects of mechanical force and tissue stiffness during development of the inner ear sensory organs. Here we describe a method for three-dimensional organotypic culture of the intact murine utricle and cochlea that overcomes these limitations. The technique for adjustment of a three-dimensional matrix stiffness described here permits manipulation of the elastic force opposing tissue growth. This method can therefore be used to study the role of mechanical forces during inner ear development. Additionally, the cultures permit virus-mediated gene delivery, which can be used for gain- and loss-of-function experiments. This culture method preserves innate hair cells and supporting cells and serves as a potentially superior alternative to the traditional two-dimensional culture of vestibular and auditory sensory organs.

Is oval window transport a royal gate for nanoparticle delivery to vestibule in the inner ear?

Drug delivery to the inner ear by nanomedicine strategies has emerged as an effective therapeutic approach for the management of inner ear diseases including hearing and balance disorders. It is well accepted that substance enters the perilymph from the middle ear through the round window membrane (RWM), but the passage through the oval window (OW) has long been neglected. Up to now, researchers still know little about the pathway via which nanoparticles (NPs) enter the inner ear or how they reach the inner ear following local applications. Herein, we engineered fluorescence traceable chitosan (CS) NPs, investigated the NP distribution within cochlear and vestibular organs, and assessed the availability of RWM and OW pathways to NP transport. Intriguingly, there were high levels of CS NPs in vestibular hair cells, dark cells and supporting cells, but negligible ones in cochlear hair cells and epithelial cells after intratympanic administration. However, the NPs were visualized in two cell models, L929 and HEI-OC1 cell lines, and in the hair cells of cochlear explants after co-incubation in vitro. These combined studies implied that CS NPs might enter the vestibule directly through the OW and then preferentially accumulated in the cells of vestibular organs. Thus, in vivo studies were carried out and clearly revealed that CS NPs entered the inner ear through both the RWM and OW, but the latter played a governing role in delivering NPs to the vestibule with vivid fluorescence signals in the thin bone of the stapes footplate. Overall, these findings firstly suggested that the OW, as a royal gate, afforded a convenient access to facilitate CS NPs transport into inner ear, casting a new light on future clinical applications of NPs in the effective treatment of vestibular disorders by minimizing the risk of hearing loss associated with cochlear hair cell pathology.

Acute blockade of inner ear marginal and dark cell K+ secretion: Effects on gravity receptor function

Specific pharmacological blockade of KCNQ (Kv7) channels with XE991 rapidly (within 20 min) and profoundly alters inner ear gravity receptor responses to head motion (Lee et al., 2017). We hypothesized that these effects were attributable to the suppression of K⁺ secretion following blockade of KCNQ1-KCNE1 channels in vestibular dark cells and marginal cells. To test this hypothesis, K⁺ secretion was independently inhibited by blocking the Na⁺-K⁺-2Cl^- cotransporter (NKCC1, Slc12a2) rather than KCNQ1-KCNE1 channels. Acute blockade of NKCC1 with ethacrynic acid (40 mg/kg) eliminated auditory responses (ABRs) within approximately 70 min of injection, but had no effect on vestibular gravity receptor function (VsEPs) over a period of 2 h in the same animals. These findings show that, vestibular gravity receptors are highly resistant to acute disruption of endolymph secretion unlike the auditory system. Based on this we argue that acute suppression of K⁺ secretion alone does not likely account for the rapid profound effects of XE991 on gravity receptors. Instead the effects of XE991 likely require additional action at KCNQ channels located within the sensory epithelium itself.

Apico-basal Polarity Determinants Encoded by crumbs Genes Affect Ciliary Shaft Protein Composition, IFT Movement Dynamics, and Cilia Length

One of the most obvious manifestations of polarity in epithelia is the subdivision of the cell surface by cell junctions into apical and basolateral domains. crumbs genes are among key regulators of this form of polarity. Loss of crumbs function disrupts the apical cell junction belt and crumbs overexpression expands the apical membrane size. Crumbs proteins contain a single transmembrane domain and localize to cell junction area at the apical surface of epithelia. In some tissues, they are also found in cilia. To test their role in ciliogenesis, we investigated mutant phenotypes of zebrafish crumbs genes. In zebrafish, mutations of three crumbs genes, oko meduzy/crb2a, crb3a, and crb2b, affect cilia length in a subset of tissues. In oko meduzy (ome), this is accompanied by accumulation of other Crumbs proteins in the ciliary compartment. Moreover, intraflagellar transport (IFT) particle components accumulate in the ciliary shaft of ome;crb3a double mutants. Consistent with the above, Crb3 knockdown in mammalian cells affects the dynamics of IFT particle movement. These findings reveal crumbs-dependent mechanisms that regulate the localization of ciliary proteins, including Crumbs proteins themselves, and show that crumbs genes modulate intraflagellar transport and cilia elongation.

Effects of gravity changes on gene expression of BDNF and serotonin receptors in the mouse brain

Spaceflight entails various stressful environmental factors including microgravity. The effects of gravity changes have been studied extensively on skeletal, muscular, cardiovascular, immune and vestibular systems, but those on the nervous system are not well studied. The alteration of gravity in ground-based animal experiments is one of the approaches taken to address this issue. Here we investigated the effects of centrifugation-induced gravity changes on gene expression of brain-derived neurotrophic factor (BDNF) and serotonin receptors (5-HTRs) in the mouse brain. Exposure to 2g hypergravity for 14 days showed differential modulation of gene expression depending on regions of the brain. BDNF expression was decreased in the ventral hippocampus and hypothalamus, whereas increased in the cerebellum. 5-HT1BR expression was decreased in the cerebellum, whereas increased in the ventral hippocampus and caudate putamen. In contrast, hypergravity did not affect gene expression of 5-HT1AR, 5-HT2AR, 5-HT2CR, 5-HT4R and 5-HT7R. In addition to hypergravity, decelerating gravity change from 2g hypergravity to 1g normal gravity affected gene expression of BDNF, 5-HT1AR, 5-HT1BR, and 5-HT2AR in various regions of the brain. We also examined involvement of the vestibular organ in the effects of hypergravity. Surgical lesions of the inner ear's vestibular organ removed the effects induced by hypergravity on gene expression, which suggests that the effects of hypergravity are mediated through the vestibular organ. In summary, we showed that gravity changes induced differential modulation of gene expression of BDNF and 5-HTRs (5-HT1AR, 5-HT1BR and 5-HT2AR) in some brain regions. The modulation of gene expression may constitute molecular bases that underlie behavioral alteration induced by gravity changes.

Perilymph pharmacokinetics of locally-applied gentamicin in the guinea pig

Intratympanic gentamicin therapy is widely used clinically to suppress the vestibular symptoms of Meniere's disease. Dosing in humans was empirically established and we still know remarkably little about where gentamicin enters the inner ear, where it reaches in the inner ear and what time course it follows after local applications. In this study, gentamicin was applied to the round window niche as a 20 μL bolus of 40 mg/ml solution. Ten 2 μL samples of perilymph were collected sequentially from the lateral semi-circular canal (LSCC) at times from 1 to 4 h after application. Gentamicin concentration was typically highest in samples originating from the vestibule and was lower in samples originating from scala tympani. To interpret these results, perilymph elimination kinetics for gentamicin was quantified by loading the entire perilymph space by injection at the LSCC with a 500 μg/ml gentamicin solution followed by sequential perilymph sampling from the LSCC after different delay times. This allowed concentration decline in perilymph to be followed with time. Gentamicin was retained well in scala vestibuli and the vestibule but declined rapidly at the base of scala tympani, dominated by interactions of perilymph with CSF, as reported for other substances. Quantitative analysis, taking into account perilymph kinetics for gentamicin, showed that more gentamicin entered at the round window membrane (57%) than at the stapes (35%) but the lower concentrations found in scala tympani were due to greater losses there. The gentamicin levels found in perilymph of the vestibule, which are higher than would be expected from round window entry alone, undoubtedly contribute to the vestibulotoxic effects of the drug. Furthermore, calculations of gentamicin distribution following targeted applications to the RW or stapes are more consistent with cochleotoxicity depending on the gentamicin concentration in scala vestibuli rather than that in scala tympani.

Ion channel proteins in mouse and human vestibular tissue

BACKGROUND AND OBJECTIVE: Electrical activity in hair cells and neurons of the inner ear is necessary for the transduction and modulation of stimuli that impinge on the cochlea and vestibular endorgans of the inner ear. The underlying basis of this activity is pore-forming proteins in the membrane of excitable cells that allow the influx and efflux of various ions, including Na+, Ca2+, and K+, among others. These channels are critical to both electrical activity as well as the development of excitable cells because they may initiate long-term signals that are important in the maintenance and survival of these cells. We investigated the expression of several Shaker potassium ion channel proteins and an accessory β subunit in the vestibular endorgans of mouse and human.

METHODS: Vestibular tissue consisting of cristae ampullares was harvested from adult and neonatal mice as well as from human subjects undergoing vestibular surgery. Western blot analysis and immunoprecipitation were used to identify the presence or absence, in mouse, of α subunits Kv1.2, Kv1.4, and Kv1.5 and of β subunit Kvβ1.1 in mouse. Coimmunoprecipitation was used to identify interactions between α and β subunits. Immunohistochemistry was used to localize Kv1.2 in mouse and human tissues.

RESULTS: The presence of Kvα1.2 and Kvβ1.1 was confirmed in adult mouse crista ampullaris by Western blotting. Coimmunoprecipitation experiments showed that Kv1.2 and Kvβ1.1 interact in these tissues. Immunostaining localized Kv1.2 to regions within and extraneous to the sensory epithelium of mouse and human cristae ampullares. In comparison, Kv1.4 and Kv1.5 were not found in the crista ampullaris.

CONCLUSIONS: We describe the presence, location, and interaction of various potassium ion channel α subunits and a β subunit. These data are initial descriptions of potassium ion channels in the mammalian vestibular system and begin to provide an understanding of the protein subunits that form ion channels of the mammalian inner ear. In addition, our data show that there are interactions that occur that may regulate the biophysical properties of these channels, thereby contributing to the diversity of channel function. This knowledge is critical to understanding the genes that encode these channels and finding cures for pathologies of hearing and balance.

SIGNIFICANCE: We detail initial characteristics of potassium ion channel proteins including α subunits Kv1.2, Kv1.4, and Kv1.5 and β subunit Kvβ1.1 in mammalian vestibular tissue. This knowledge is critical to understanding the processing of vestibular stimuli and the regulation of endolymphatic function. Mutations of ion channels can cause neurological pathologies including auditory and vestibular disorders in humans.

Molecular aging of the mammalian vestibular system

Dizziness and imbalance frequently affect the elderly and contribute to falls and frailty. In many geriatric patients, clinical testing uncovers a dysfunction of the vestibular system, but no specific etiology can be identified. Neuropathological studies have demonstrated age-related degeneration of peripheral and central vestibular neurons, but the molecular mechanisms are poorly understood. In contrast, recent studies into age-related hearing loss strongly implicate mitochondrial dysfunction, oxidative stress and apoptotic cell death of cochlear hair cells. While some data suggest that analogous biological pathomechanisms may underlie vestibular dysfunction, actual proof is missing. In this review, we summarize the available data on the molecular causes of vestibular dysfunction.

Anatomical and physiological development of the human inner ear

We describe the development of the human inner ear with the invagination of the otic vesicle at 4 weeks gestation (WG), the growth of the semicircular canals from 5 WG, and the elongation and coiling of the cochlea at 10 WG. As the membranous labyrinth takes shape, there is a concomitant development of the sensory neuroepithelia and their associated structures within. This review details the growth and differentiation of the vestibular and auditory neuroepithelia, including synaptogenesis, the expression of stereocilia and kinocilia, and innervation of hair cells by afferent and efferent nerve fibres. Along with development of essential sensory structures we outline the formation of crucial accessory structures of the vestibular system - the cupula and otolithic membrane and otoconia as well as the three cochlea compartments and the tectorial membrane. Recent molecular studies have elaborated on classical anatomical studies to characterize the development of prosensory and sensory regions of the fetal human cochlea using the transcription factors, PAX2, MAF-B, SOX2, and SOX9. Further advances are being made with recent physiological studies that are beginning to describe when hair cells become functionally active during human gestation. This article is part of a Special Issue entitled <Annual Reviews 2016>.

Neurotransmitters in the vestibular system

Neuronal networks that are linked to the peripheral vestibular system contribute to gravitoinertial sensation, balance control, eye movement control, and autonomic function. Ascending connections to the limbic system and cerebral cortex are also important for motion perception and threat recognition, and play a role in comorbid balance and anxiety disorders. The vestibular system also shows remarkable plasticity, termed vestibular compensation. Activity in these networks is regulated by an interaction between: (1) intrinsic neurotransmitters of the inner ear, vestibular nerve, and vestibular nuclei; (2) neurotransmitters associated with thalamocortical and limbic pathways that receive projections originating in the vestibular nuclei; and (3) locus coeruleus and raphe (serotonergic and nonserotonergic) projections that influence the latter components. Because the ascending vestibular interoceptive and thalamocortical pathways include networks that influence a broad range of stress responses (endocrine and autonomic), memory consolidation, and cognitive functions, common transmitter substrates provide a basis for understanding features of acute and chronic vestibular disorders.

Efferent Vestibular Neurons Show Homogenous Discharge Output But Heterogeneous Synaptic Input Profile In Vitro

Despite the importance of our sense of balance we still know remarkably little about the central control of the peripheral balance system. While previous work has shown that activation of the efferent vestibular system results in modulation of afferent vestibular neuron discharge, the intrinsic and synaptic properties of efferent neurons themselves are largely unknown. Here we substantiate the location of the efferent vestibular nucleus (EVN) in the mouse, before characterizing the input and output properties of EVN neurons in vitro. We made transverse serial sections through the brainstem of 4-week-old mice, and performed immunohistochemistry for calcitonin gene-related peptide (CGRP) and choline acetyltransferase (ChAT), both expressed in the EVN of other species. We also injected fluorogold into the posterior canal and retrogradely labelled neurons in the EVN of ChAT:: tdTomato mice expressing tdTomato in all cholinergic neurons. As expected the EVN lies dorsolateral to the genu of the facial nerve (CNVII). We then made whole-cell current-, and voltage-clamp recordings from visually identified EVN neurons. In current-clamp, EVN neurons display a homogeneous discharge pattern. This is characterized by a high frequency burst of action potentials at the onset of a depolarizing stimulus and the offset of a hyperpolarizing stimulus that is mediated by T-type calcium channels. In voltage-clamp, EVN neurons receive either exclusively excitatory or inhibitory inputs, or a combination of both. Despite this heterogeneous mixture of inputs, we show that synaptic inputs onto EVN neurons are predominantly excitatory. Together these findings suggest that the inputs onto EVN neurons, and more specifically the origin of these inputs may underlie EVN neuron function.

Contribution of vestibular efferent system alpha-9 nicotinic receptors to vestibulo-oculomotor interaction and short-term vestibular compensation after unilateral labyrinthectomy in mice

Sudden unilateral loss of vestibular afferent input causes nystagmus, ocular misalignment, postural instability and vertigo, all of which improve significantly over the first few days after injury through a process called vestibular compensation (VC). Efferent neuronal signals to the labyrinth are thought to be required for VC. To better understand efferent contributions to VC, we compared the time course of VC in wild-type (WT) mice and α9 knockout (α9(-/-)) mice, the latter lacking the α9 subunit of nicotinic acetylcholine receptors (nAChRs), which is thought to represent one signaling arm activated by the efferent vestibular system (EVS). Specifically, we investigated the time course of changes in the fast/direct and slow/indirect components of the angular vestibulo-ocular reflex (VOR) before and after unilateral labyrinthectomy (UL). Eye movements were recorded using infrared video oculography in darkness with the animal stationary and during sinusoidal (50 and 100°/s, 0.5-5 Hz) and velocity step (150°/s for 7-10s, peak acceleration 3000°/s(2)) passive whole-body rotations about an Earth-vertical axis. Eye movements were measured before and 0.5, 2, 4, 6 and 9 days after UL. Before UL, we found frequency- and velocity-dependent differences between WT and α9(-/-) mice in generation of VOR quick phases. The VOR slow phase time constant (TC) during velocity steps, which quantifies contributions of the indirect component of the VOR, was longer in α9(-/-) mutants relative to WT mice. After UL, spontaneous nystagmus (SN) was suppressed significantly earlier in WT mice than in α9(-/-) mice, but mutants achieved greater recovery of TC symmetry and VOR quick phases. These data suggest (1) there are significant differences in vestibular and oculomotor functions between these two types of mice, and (2) efferent signals mediated by α9 nicotinic AChRs play a role during VC after UL.

Block of NMDA receptor channels by endogenous neurosteroids: implications for the agonist induced conformational states of the channel vestibule

N-methyl-D-aspartate receptors (NMDARs) mediate synaptic plasticity, and their dysfunction is implicated in multiple brain disorders. NMDARs can be allosterically modulated by numerous compounds, including endogenous neurosteroid pregnanolone sulfate. Here, we identify the molecular basis of the use-dependent and voltage-independent inhibitory effect of neurosteroids on NMDAR responses. The site of action is located at the extracellular vestibule of the receptor's ion channel pore and is accessible after receptor activation. Mutations in the extracellular vestibule in the SYTANLAAF motif disrupt the inhibitory effect of negatively charged steroids. In contrast, positively charged steroids inhibit mutated NMDAR responses in a voltage-dependent manner. These results, in combination with molecular modeling, characterize structure details of the open configuration of the NMDAR channel. Our results provide a unique opportunity for the development of new therapeutic neurosteroid-based ligands to treat diseases associated with dysfunction of the glutamate system.

Cochlin expression in vestibular endorgans obtained from patients with Meniere's disease

The distribution of cochlin and its associated basement membrane proteins (collagen IV, collagen II, laminin-β2, and nidogen-1) were evaluated in the vestibular endorgans of subjects with Meniere's disease and compared with normal specimens. Cochlin mRNA expression in vestibular endorgans from Meniere's disease specimens was also investigated. Specimens were obtained from patients who had Meniere's disease and who were undergoing ablative labyrinthectomy. Control specimens were obtained both from autopsy specimens with documented normal audiovestibular function and from patients undergoing labyrinthectomy for acoustic neuroma excision. In the normal control specimens, cochlin immunoreactivity was found evenly distributed in the stroma of the cristae ampullaris and maculae of the utricle. In Meniere's specimens, cochlin immunoreactivity was markedly increased; this was associated with an increase in cochlin mRNA expression as shown by real-time reverse transcription with the polymerase chain reaction. Collagen IV and laminin-β2 immunoreactivity was significantly decreased in Meniere's specimens. Nidogen-1 and collagen II immunoreactivity was unchanged in Meniere's specimens when compared with normal samples. Cochlin upregulation has been implicated in the hereditary audiovestibulopathy, DFNA9. The increased expression of cochlin and decreased expression of collagen IV and laminin in Meniere's disease are suggestive that the overexpression of cochlin contributes to the dysfunctional inner ear homeostasis seen in this disease.

Immunoelectron microscopic analysis of neurotoxic effect of glutamate in the vestibular end organs during ischemia

CONCLUSION

The excessive glutamate released from the type I and type II hair cells and the supporting cells injure the bouton-type endings and the nerve chalices in 30 min ischemia, and neuronal damage of glutamate was slight in 10 min ischemia.

OBJECTIVE

In the present study, we investigated by means of post-embedding immunoelectron microscopic analysis whether neuronal damage in the vestibular end organs is associated with the change of cellular glutamate concentration during ischemia.

METHODS

Transient local anoxia (10 min, 30 min) of guinea pig inner ear was induced by pressing the left labyrinthine artery. The right sides were used as controls. The morphological changes of the vestibular end organs and the areal gold particle densities representing glutamate were compared in the ischemia side and the control side.

RESULTS

The areal gold particle densities of the type I and type II hair cells and the supporting cells in the ischemic side were lower than those of the control side. There were no remarkable morphological changes compared to the control side in 10 min ischemia. In 30 min ischemia, the bouton-type endings were swollen and intercellular spaces between the type I hair cells and the nerve chalices were enlarged.

Roles of the espin actin-bundling proteins in the morphogenesis and stabilization of hair cell stereocilia revealed in CBA/CaJ congenic jerker mice

Hearing and vestibular function depend on mechanosensory staircase collections of hair cell stereocilia, which are produced from microvillus-like precursors as their parallel actin bundle scaffolds increase in diameter and elongate or shorten. Hair cell stereocilia contain multiple classes of actin-bundling protein, but little is known about what each class contributes. To investigate the roles of the espin class of actin-bundling protein, we used a genetic approach that benefited from a judicious selection of mouse background strain and an examination of the effects of heterozygosity. A congenic jerker mouse line was prepared by repeated backcrossing into the inbred CBA/CaJ strain, which is known for excellent hearing and minimal age-related hearing loss. We compared stereocilia in wild-type CBA/CaJ mice, jerker homozygotes that lack espin proteins owing to a frameshift mutation in the espin gene, and jerker heterozygotes that contain reduced espin levels. The lack of espins radically impaired stereociliary morphogenesis, resulting in stereocilia that were abnormally thin and short, with reduced differential elongation to form a staircase. Mean stereociliary diameter did not increase beyond ∼0.10–0.14 µm, making stereocilia ∼30%–60% thinner than wild type and suggesting that they contained ∼50%–85% fewer actin filaments. These characteristics indicate a requirement for espins in the appositional growth and differential elongation of the stereociliary parallel actin bundle and fit the known biological activities of espins in vitro and in transfected cells. The stereocilia of jerker heterozygotes showed a transient proximal-distal tapering suggestive of haploinsufficiency and a slowing of morphogenesis that revealed previously unrecognized assembly steps and intermediates. The lack of espins also led to a region-dependent degeneration of stereocilia involving shortening and collapse. We conclude that the espin actin-bundling proteins are required for the assembly and stabilization of the stereociliary parallel actin bundle.

Stereocilia are the fingerlike projections of inner ear hair cells that detect sound and motion. Stereocilia grow to specific lengths and diameters and form staircase-like arrays. The changes in size appear to be driven by matching alterations in the dimensions of an underlying molecular scaffold consisting of a bundle of actin filaments cross-linked by actin-bundling proteins. To elucidate the roles of the espin actin-bundling proteins in hair cell stereocilia, we carry out an in-depth accounting of stereociliary size and shape in the jerker mutant mouse, which lacks the espin proteins because of a mutation in the espin gene. We examine a new and improved jerker mouse with a genetic background known for high-quality lifelong hearing. We find that, in the absence of espins, stereocilia do not increase in diameter or complete their elongation, but instead bend, shorten, and disappear. Although the specifics vary according to inner ear region, the stereociliary defects are profound and can readily account for the deafness and balance problems of jerker mice and humans with certain espin gene mutations. Even reducing espin levels by one-half leads to temporary defects in stereociliary diameter. Thus, espins play crucial roles in the formation and maintenance of hair cell stereocilia.

The effect of changes in perilymphatic K+ on the vestibular evoked potential in the guinea pig

To investigate the effect on the functioning of the vestibular system of a rupture of Reissner's membrane, artificial endolymph was injected in scala media of ten guinea pigs and vestibular evoked potentials (VsEPs), evoked by vertical acceleration pulses, were measured. Directly after injection of a sufficient volume to cause rupture, all ears showed a complete disappearance of VsEP, followed by partial recovery. To investigate the effect of perilymphatic potassium concentration on the vestibular sensory and neural structures, different concentrations of KCl were injected directly into the vestibule. The KCl injections resulted in a dose-dependent decrease of VsEP, followed by a dose-dependent slow recovery. This animal model clearly shows a disturbing effect of a higher than normal K(+) concentration in perilymph on the vestibular and neural structures in the inner ear. Potassium intoxication is the most probable explanation for the observed effects. It is one of the explanations for Menière attacks.

Neurofilament localization and phosphorylation in the developing inner ear of the rat

Detailed understanding of neurofilament protein distribution in the inner ear can shed light on regulatory mechanisms involved in neuronal development of this tissue. We assessed the spatio-temporal changes in the distribution of neurofilaments in the developing rat inner ear between embryonic day 12 and 30 days after birth, using antibodies against phosphorylated as well as non-phosphorylated light (NFL), medium (NFM) and heavy (NFH) neurofilament subunits. Our results show that during development, the onset of neurofilament expression in the rat inner ear is on embryonic day 12, earlier than previously shown. We demonstrate that neurofilament subunits of different molecular weight emerge in a developmental stage-dependent order. In addition, we determined that neurofilaments of the vestibular nerve mature earlier than neurofilaments of the cochlear nerve. Cochlear neurofilament maturation progresses in a gradient from base to apex, and from inner to outer hair cells. The sequential pattern of neurofilament expression we describe may help understand the consequences of certain mutations, and contribute to develop therapeutic strategies.

Canal cristae growth and fiber extension to the outer hair cells of the mouse ear require Prox1 activity

BACKGROUND

The homeobox gene Prox1 is required for lens, retina, pancreas, liver, and lymphatic vasculature development and is expressed in inner ear supporting cells and neurons.

METHODOLOGY/PRINCIPAL FINDINGS

We have investigated the role of Prox1 in the developing mouse ear taking advantage of available standard and conditional Prox1 mutant mouse strains using Tg(Pax2-Cre) and Tg(Nes-Cre). A severe reduction in the size of the canal cristae but not of other vestibular organs or the cochlea was identified in the E18.5 Prox1(Flox/Flox); Tg(Pax2-Cre) mutant ear. In these mutant embryos, hair cell differentiated; however, their distribution pattern was slightly disorganized in the cochlea where the growth of type II nerve fibers to outer hair cells along Prox1 expressing supporting cells was severely disrupted. In the case of Nestin-Cre, we found that newborn Prox1(Flox/Flox); Tg(Nestin-Cre) exhibit only a disorganized innervation of outer hair cells despite apparently normal cellular differentiation of the organ of Corti, suggesting a cell-autonomous function of Prox1 in neurons.

CONCLUSIONS/SIGNIFICANCE

These results identify a dual role of Prox1 during inner ear development; growth of the canal cristae and fiber guidance of Type II fibers along supporting cells in the cochlea.

The possible contribution of angiitis to the onset of benign paroxysmal positional vertigo (BPPV)

OBJECTIVE

The aim was to evaluate the oxidative stress and the angiitis in patients with BPPV.

METHOD

Patients with benign paroxysmal positional vertigo (BPPV) within 14 days of onset were analyzed. The level of diacron reactive oxygen metabolites (d-ROM) and circulating soluble vascular cell adhesion molecule 1 (VCAM-1), were evaluated. As a treatment the patients were taught to perform the Brandt-Daroff exercise at home by themselves. The prognosis of BPPV, which is measured as the time until the disappearance of positional nystagmus by a physician during the outpatient visit each week, the relation among the level of oxygen metabolites, vascular molecule and the duration until remission were analyzed.

RESULTS

The patients who required longer time for the disappearance of positional nystagmus showed high d-ROM and VCAM levels, whereas those who required shorter time for remission showed lower d-ROM and VCAM levels.

CONCLUSION

There is an increased expression of VCAM-1 and d-ROM confirming the existence of an angiitis and supporting the vascular involvement in BPPV. The identification of the high levels of d-ROM and VCAM-1 can open the way to selective pharmacological treatments able to correct the oxidative stress and activation of endothelial cells.

The expression of atrial natriuretic peptide receptor in the mouse inner ear labyrinth

OBJECTIVE

To compare the relative expression levels of NPR-A mRNA in the stria vascularis (StV), nonstrial tissue of the cochlear lateral wall (NSt) and vestibula in the mouse inner ear to determine the potential contribution of ANP signaling in different parts to inner ear fluid homeostasis.

SETTING

Atrial natriuretic peptide (ANP) is a cardiac hormone known to be involved in the regulation of body fluid homeostasis. It is assumed that ANP might also participate in the regulation of inner ear fluid dynamics. ANP selectively binds with atrial natriuretic peptide receptor (NPR-A) to exert its physiological function. We have previously shown the presence of NPR-A transcripts in the mouse stria vascularis as well as in the nonstrial tissue of the cochlear lateral wall and vestibular organ by polymerase-chain reaction.

METHODS

Total RNA of samples of stria vascularis, nonstrial tissue of the cochlear lateral wall and vestibular organ tissue from ears of 10 adult mice was isolated, amplified by the real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) using consensus primers flanking a region of 127 bp at the target sequences. The brain of mice known to contain NPR-A was used as a positive control. The one-way analysis of variance, ANOVA and Student-Newman-Keuls method were performed to analyze the data.

RESULTS

NPR-A mRNA expression was found in tissue samples of all the three areas. The mRNA expression level of NPR-A in the StV was higher than that in the NSt and vestibula (p<0.05). The difference of NPR-A mRNA expression level in the NSt and vestibula was not statistically significant (p>0.05).

CONCLUSIONS

Our results suggest that the StV may be the most important place for ANP to regulate endolymph fluid balance via NPR-A.

[Expression of group I mGluRs in rat flocculus after unilateral labyrinthectomy]

OBJECTIVE

Studies revealed that cerebellar flocculus-paraflocculus complex plays an important in vestibular compensation. To observe the expression change of Group I mGluRs in flocculus following left unilateral labyrinthectomy (UL).

METHOD

After setting left labyrinthectomy, the change of Group I mGluRs was detected by RT-PCR.

RESULT

Group I mGluR5 was induced increase in both side flocculus after unilateral labyrinthectomy. By contrast, mGluR1 was induced decrease. However, that of the lesioned side was stronger than that of the unlesioned side.

CONCLUSION

UL can induce change of Group I mGluRs in the flocculus. The fall in the resting discharge of the primary vestibular afferents and/or in the deafferented central vestibular neurons may cause the change of mGluRs. But the significance of the change of mGluRs in the vestibular compensation is still unknown.

[Expression change of mGluR5 in rat MVN after unilateral labyrinthectomy]

OBJECTIVE

To observe the expression of mGluR5 in the medial vestibular nucleus (MVN) following unilateral labyrinthectomy (UL).

METHOD

Thirty Wistar rats were randomly divided into two groups. Twenty four animals received unilateral labyrinthectomy while the others maintained labyrinthine well. After setting left labyrinthine, the change of mGluR5 was induced by immunohistochemistry, in situ hybridization.

RESULT

mGluR5 was increased in lesioned side MVN after unilateral labyrinthectomy. The 12 h post-UL was highest. Then it was decrease in 36 h post-UL, while 7 d post-UL was same as control group. The change of contralateral was same as that in ipsilateral.

CONCLUSION

UL can induce increase of mGluR5 in the MVN. The reduced resting discharge in the primary vestibular afferents or in the central vestibular neurons may be responsible for the change of mGluR5. However the significance of the change of mGluR1 in the vestibular compensation is still unknown.

A late role for bmp2b in the morphogenesis of semicircular canal ducts in the zebrafish inner ear

BACKGROUND

The Bone Morphogenetic Protein (BMP) genes bmp2 and bmp4 are expressed in highly conserved patterns in the developing vertebrate inner ear. It has, however, proved difficult to elucidate the function of BMPs during ear development as mutations in these genes cause early embryonic lethality. Previous studies using conditional approaches in mouse and chicken have shown that Bmp4 has a role in semicircular canal and crista development, but there is currently no direct evidence for the role of Bmp2 in the developing inner ear.

METHODOLOGY/PRINCIPAL FINDINGS

We have used an RNA rescue strategy to test the role of bmp2b in the zebrafish inner ear directly. Injection of bmp2b or smad5 mRNA into homozygous mutant swirl (bmp2b(-/-)) embryos rescues the early patterning defects in these mutants and the fish survive to adulthood. As injected RNA will only last, at most, for the first few days of embryogenesis, all later development occurs in the absence of bmp2b function. Although rescued swirl adult fish are viable, they have balance defects suggestive of vestibular dysfunction. Analysis of the inner ears of these fish reveals a total absence of semicircular canal ducts, structures involved in the detection of angular motion. All other regions of the ear, including the ampullae and cristae, are present and appear normal. Early stages of otic development in rescued swirl embryos are also normal.

CONCLUSIONS/SIGNIFICANCE

Our findings demonstrate a critical late role for bmp2b in the morphogenesis of semicircular canals in the zebrafish inner ear. This is the first demonstration of a developmental role for any gene during post-embryonic stages of otic morphogenesis in the zebrafish. Despite differences in the early stages of semicircular canal formation between zebrafish and amniotes, the role of Bmp2 in semicircular canal duct outgrowth is likely to be conserved between different vertebrate species.

[Expression change of Muscarinic receptor subunits in rat flocculus following unilateral labyrinthectomy]

OBJECTIVE

To observe the expression of Muscarinic receptor M1, M3, M5 subunits in rat flocculus following left unilateral labyrinthectomy (UL).

METHOD

The RT-PCR was used to observe the expression of Muscarinic receptor M1, M3, M5 subunits post-unilateral labyrinthectomy and investigate its effect on vestibular compensation.

RESULT

Muscarinic receptor M1, M3, M5 subunits were induced decrease in both side flocculus after unilateral labyrinthectomy. The expression was the least in the 1 d flocculus of following UL. The expression is rising from the 3-7 d flocculus of following UL. No difference was observed in the 7 d and sham operation flocculus following UL. No difference was observed in the ipsilateral and contralateral flocculus at any group.

CONCLUSION

Muscarinic receptor M1, M3, M5 subunits were induced decrease in the flocculus after unilateral labyrinthectomy. But the significance of the change of Muscarinic receptor M1, M3, M5 subunits in the vestibular compensation is still unknown.

Detection of atrial natriuretic peptide receptor in the labyrinth of the mouse inner ear

OBJECTIVES

To examine whether the atrial natriuretic peptide receptor (NPR-A) is present in the secretory regions of the membrane labyrinth of the adult mouse inner ear.

SETTING

Recent studies have implied that the homeostasis of endolymph fluid in the inner ear may be regulated by receptor-mediated mechanisms. Several studies have identified the presence of atrial natriuretic peptide receptors in the inner ear of guinea pig and rat. As a member of the natriuretic peptide receptor family, which also includes B-type natriuretic peptide receptor (NPR-B) and C-type natriuretic peptide receptor (NPR-C), NPR-A may be involved in the regulation of fluid homeostasis in the inner ear.

METHODS

In this study, samples of stria vascularis, nonstrial tissue of the cochlear lateral wall and vestibular organ tissue from the ears of 6 adult mice were obtained by immediate excision of bony labyrinth under operating microscope after decapitation. Total RNA was isolated and mRNA was amplified by the reverse transcription-polymerase chain reaction (RT-PCR) using consensus primers flanking a region of 127 bp at the target sequence. Mouse renal cortex known to contain NPR-A was used as a positive control.

RESULTS

We demonstrated that NPR-A was expressed in the mouse stria vascularis as well as in the nonstrial tissue of the cochlear lateral wall and vestibular organ.

CONCLUSION

These results suggest that natriuretic peptides may play an important role in maintaining the fluid homeostasis of inner ear endolymph via interaction with NPR-A.

Expression of cocaine- and amphetamine-regulated transcript (CART) peptides at climbing fibre-Purkinje cell synapses in the rat vestibular cerebellum

Cocaine- and amphetamine-regulated transcript (CART) peptides have a wide CNS distribution and appear to play an important role in a number of physiological processes including reward and reinforcement, feeding, locomotion, stress responses and perception of pain. We have further investigated the expression of CART peptides in rat cerebellum, using multiple fluorescent antibodies. Dense fibre-like immunofluorescence was observed in the molecular layer of the vestibular cerebellum (paraflocculus and lobes IX and X of the vermis). There was little or no immunofluorescence in any other region of the cerebellum (vermis or hemispheres). The immunofluorescence in lobes IX and X of the vermis showed a parasagittal banding pattern, with one medial and two lateral bands. We have provided several lines of evidence that CART peptides are expressed by climbing fibres: (1) There is CART peptide immunofluorescence in the inferior olivary complex, the source of climbing fibres. (2) No cerebellar cell bodies were labelled by the CART peptide antibody. (3) The developmental profile of CART peptide expression was consistent with climbing fibre development and finally (4) triple antibody labelling revealed co-expression of CART peptides with VGluT2 (the glutamate transporter at climbing fibre synapses) at boutons innervating calbindin D-28K labelled proximal Purkinje cell dendrites. Analysis of VGluT2 and CART peptide labelling in more detail showed that not all climbing fibres in the vestibular cerebellum expressed CART peptides but those fibres which did express CART peptides were immunofluorescent at all Purkinje cell synapses.

Kvbeta1.1 associates with Kvalpha1.4 in Chinese hamster ovary cells and pigeon type II vestibular hair cells and enhances the amplitude, inactivation and negatively shifts the steady-state inactivation range

Although A-type potassium currents are found in type II hair cells in the inner ear of most species, the molecular mechanisms for activation and inactivation of the A-type potassium current (I(A)) remain unknown. In frog semicircular canal hair cells, for example, there appear to be two classes of currents having either fast or slow inactivation [Norris CH, Ricci AJ, Housley GD, Guth PS (1992) The inactivating potassium currents of hair cells isolated from the crista ampullaris of the frog. J Neurophysiol 68:1642-1653; Russo G, Calzi D, Martini M, Rossi ML, Fesce R, Prigioni I (2007) Potassium currents in the hair cells of vestibular epithelium: position-dependent expression of two types of A channels. Eur J Neurosci 25:695-704]. It has been suggested that somehow the "ball and chain" mechanism (NH(3) (N) terminus motif) is modified by alternative splicing to account for the two classes of inactivation. To examine other possibilities, we cloned alpha and beta subunits that comprise the A-type potassium channel complex in adult and embryonic pigeon brain, cochlea and labyrinth. By sequence homology, we concluded that the subunits present were Kvalpha1.4 and Kvbeta1.1. The sequence of the open reading frame for Kvalpha1.4 contained the N-terminus, pore and COOH (C) terminus motifs for N-and C-type inactivation. The sequence for Kvbeta1.1 displayed amino acids consistent with assembly and association with Kvalpha1.4 alpha subunits. Kvalpha1.4 and Kvbeta1.1 were transfected either singly or in combination into Chinese hamster ovary (CHO) cells. These cells and native hair cells from the pigeon utricle were patch clamped and the inactivation properties of the A-type current were studied. In the native hair cells, the A-type current was identified by its pharmacological (4-aminopyridine (4-AP); IC(50)=11 microM) and voltage dependent inactivation properties. A comparison of the mean time constants from best-fitted single exponential and sum of two exponential equations to the ionic current inactivation revealed the following. In CHO cells when Kvalpha1.4 was expressed alone, the mean time constant (tau(1)=107 ms+/-19, N=32) was significantly (P<0.001) longer and the mean peak amplitude (2.28 nA+/-0.39, N=32) was smaller than when Kvalpha1.4 and Kvbeta1.1 were expressed in CHO cells. Moreover, the co-transfection of Kvalpha1.4 and Kvbeta1.1 into CHO cells caused a shift in the steady state inactivation curve parameter Vo 30 mV in the hyperpolarized direction relative to CHO cells expressing only Kvalpha1.4. Similarly, Kvalpha1.4-transfected CHO cells produced longer time constants and smaller amplitudes than those found for native utricular hair cells. These data lead us to conclude that while the amino acid motifs are present in Kvalpha1.4 and Kvbeta1.1 to suggest N-and C-type inactivation, co-assembly and association of Kvalpha1.4 and Kvbeta1.1 may also produce changes in the time dependent inactivation properties of vestibular hair cells.

Intratympanic injection of dexamethasone: time course of inner ear distribution and conversion to its active form

HYPOTHESIS

Intratympanically injected dexamethasone 21-phosphate is converted to its active form dexamethasone in the inner ear and follows the distribution of the glucocorticoid receptor.

BACKGROUND

Although dexamethasone is routinely delivered intratympanically for hearing loss, we know little of its inner ear pharmacokinetics. Dexamethasone 21-phosphate is the pharmaceutical compound available for injection, but it must be converted to its biologically active form (dexamethasone) to bind to the glucocorticoid receptor. Therefore, the current study was conducted to determine the time course of dexamethasone 21-phosphate movement from the middle ear into the inner ear, its conversion to dexamethasone, and the distribution of both forms relative to the glucocorticoid receptor.

METHODS

BALB/c mice were injected intratympanically with the prodrug dexamethasone 21-phosphate and inner ears collected at postinjection times ranging from 5 minutes to 7 days. Ears were immunohistochemically stained for dexamethasone 21-phosphate, dexamethasone, and the glucocorticoid receptor.

RESULTS

Both forms of dexamethasone were seen in the inner ear within 15 minutes, reaching their highest staining intensity at 1 hour. Neither drug was seen after 24 hours. The strongest staining occurred in the spiral ligament, organ of Corti, spiral ganglion, and vestibular sensory epithelia. Distribution of the drug paralleled locations of the glucocorticoid receptor except in the stria vascularis marginal cells, which stained heavily for the receptor but not the drug.

CONCLUSION

Dexamethasone rapidly travels from the middle ear into the inner ear and converts to its active form. The drug distribution follows that of the glucocorticoid receptor. However, it probably has little impact on ear tissues after 24 hours.

[Comparative analysis of the effect of endogenous antibiotic defensin NP-1 and aminoglycoside antibiotic gentamicin on synaptic transmission in receptors of the frog vestibular apparatus]

The effect of human and rabbit neutrophilic defensins NP-1 and amonoglycoside antibiotic gentamicin on the synaptic transmission in the afferent synapse of isolated vestibular apparatus of the frog has been comparatively studied. Both defensins proved active in the concentration range of 0.0001 to 1 nM and efficiently decreased the impulse frequency in the afferent nerve fibers in a concentration-dependent manner. No significant differences in the efficiency of rabbit and human defensin NP-1 have been revealed in these experiments. Gentamicin also had an inhibitory effect on the afferent discharge in the concentration range of 10-500 microM (0.5-25 mg/kg). The inhibitory effect of gentamicin on the impulse activity of the vestibular nerve was observed at therapeutic doses. The excitatory effect of the putative neurotransmitter L-glutamate was considerably inhibited by defensin NP-1. These findings suggest that the mechanism of defensin action involves a modification of the synaptic transmission the hair receptor and is mediated by L-glutamate.

Localization of vesicular glutamate transporters in the peripheral vestibular system of rat

OBJECTIVE

To examine the vesicular glutamate transporters (VGluTs: VGluT1-VGluT3) in the peripheral vestibular system.

METHODS

The vestibular structures, including Scarpa's ganglion (vestibular ganglion, VG), maculae of utricle and saccule, and ampullary cristae, from normal Sprague-Dawley rats were processed immunohistochemically for VGluTs, by avidin-biotinylated peroxidase complex method, with 3-3'-diaminobenzidine (DAB) as chromogen.

RESULTS

(1) VGluT1 was localized to partial neurons of VG and to the putative primary afferent fibers innervating vestibular end-organs. (2) Intense VGluT3 immunoreactivity was detected in large number of sensory epithelia cells, and weak labeling of VGluT3-positive afferent fibers was in the maculae and ampullary cristae. (3) No or very weak VGluT2 immunoreactivity was observed in the VG and acoustic maculae.

CONCLUSION

These results provide the morphological support that glutamate exists in the peripheral vestibular system, and it may play an important role in the centripetal vestibular transmission.

The micromachinery of mechanotransduction in hair cells

Mechanical stimuli generated by head movements and changes in sound pressure are detected by hair cells with amazing speed and sensitivity. The mechanosensitive organelle, the hair bundle, is a highly elaborated structure of actin-based stereocilia arranged in precise rows of increasing height. Extracellular linkages contribute to its cohesion and convey forces to mechanically gated channels. Channel opening is nearly instantaneous and is followed by a process of sensory adaptation that keeps the channels poised in their most sensitive range. This process is served by motors, scaffolds, and homeostatic mechanisms. The molecular constituents of this process are rapidly being elucidated, especially by the discovery of deafness genes and antibody targets.

The role of defensins in the excitability of the peripheral vestibular system in the frog: Evidence for the presence of communication between the immune and nervous systems

Defensins are one of the major groups of endogenous peptides that are considered to be important antibiotic-like effectors of host innate and adaptive antimicrobial immunity. The current study investigated the electrophysiological effects of externally applied human and rabbit defensins (HNP-1 and RNP-1, correspondingly) on afferent neurotransmission in the frog semicircular canals (SCC). Application of HNP-1 and RNP-1 induces a concentration-dependent decrease in resting activity. Threshold concentrations for both substances were of the order of 0.0001 nM. The firing evoked by L-glutamate (L-Glu) and its agonists alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) and (1S, 3R)-1-aminocyclopentane-trans-1,3-dicarboxilic acid (ACPD) could be inhibited by HNP-1, suggesting that defensins exert inhibitory control over both ionotropic and metabotropic glutamate receptors. HNP-1 considerably inhibited the L-glutamate/high Mg2+ -induced increase in frequency, thus, demonstrating its postsynaptic site of action. Acetylcholine (ACh) responses under HNP-1 did not differ from the frequency increase induced by ACh alone, and the ACh antagonist atropine left the response to HNP-1 intact. The specific opioid receptor antagonist naloxone (Nal) antagonized the inhibitory response evoked by HNP-1. The results obtained support the evidence for the recruitment of defensins in communication between the immune and nervous systems, and on the potential of sensory receptors to participate in the inflammatory response.

An in vitro model system to study gene therapy in the human inner ear

The confined fluid-filled labyrinth of the human inner ear presents an opportunity for introduction of gene therapy reagents designed to treat hearing and balance dysfunction. Here we present a novel model system derived from the sensory epithelia of human vestibular organs and show that the tissue can survive up to 5 days in vitro. We generated organotypic cultures from 26 human sensory epithelia excised at the time of labyrinthectomy for intractable Meniere's disease or vestibular schwannoma. We applied multiply deleted adenoviral vectors at titers between 10(5) and 10(8) viral particles/ml directly to the cultures for 4-24 h and examined the tissue 12-96 h post-transfection. We noted robust expression of the exogenous transgene, green fluorescent protein (GFP), in hair cells and supporting cells suggesting both were targets of adenoviral transfection. We also transfected cultures with a vector that carried the genes for GFP and KCNQ4, a potassium channel subunit that causes dominant-progressive hearing loss when mutated. We noted a positive correlation between GFP fluorescence and KCNQ4 immunolocalization. We conclude that our in vitro model system presents a novel and effective experimental paradigm for evaluation of gene therapy reagents designed to restore cellular function in patients who suffer from inner ear disorders.

Immunocytochemical localization of ubiquitin A-52 protein in the mouse inner ear

The ubiquitin A-52 residue ribosomal protein fusion product 1 (UbA52) is a gene highly expressed specifically in the inner ear. Through cellular localization we immunocytochemically investigated its function in the inner ear. In the adult mouse, UbA52 protein was distributed in the strial marginal cells and vestibular dark cells, which regulate the endolymphatic ion homeostasis. In the developing mouse cochlea, no significant staining was observed from birth to postnatal day 3, whereas after postnatal day 6, strong UbA52-immunoreactivities were observed in strial marginal cells. Endolymphatic K concentration is elevated between postnatal days 3-8: therefore, our results indicate that UbA52 may have a functional role in regulation of ion secretion in the inner ear.

Thyroid hormone receptor alpha1 is a critical regulator for the expression of ion channels during final differentiation of outer hair cells

Cochlear outer hair cells (OHCs) terminally differentiate prior to the onset of hearing. During this time period, thyroid hormone (TH) dramatically influences inner ear development. It has been shown recently that TH enhances the expression of the motor protein prestin via liganded TH receptor beta (TRbeta) while in contrast the expression of the potassium channel KCNQ4 is repressed by unliganded TRalpha1. These different mechanisms of TH regulation by TRalpha1 or TRbeta prompted us to analyse other ion channels that are required for the final differentiation of OHCs. We analysed the onset of expression of the Ca(2+) channel Ca(V)1.3, and the K(+) channels SK2 and BK and correlated the results with the regulation via TRalpha1 or TRbeta. The data support the hypothesis that proteins expressed in rodents prior to or briefly after birth like Ca(V)1.3 and prestin are either independent of TH (e.g. Ca(V)1.3) or enhanced through TRbeta (e.g. prestin). In contrast, proteins expressed in rodents later than P6 like KCNQ4 ( approximately P6), SK2 ( approximately P9) and BK ( approximately P11) are repressed through TRalpha1. We hypothesise that the precise regulation of expression of the latter genes requires a critical local TH level to overcome the TRalpha1 repression.

Differential expression of otoferlin in brain, vestibular system, immature and mature cochlea of the rat

Mutations of the human otoferlin gene lead to an autosomal recessive nonsyndromic form of prelingual, sensorineural deafness (deafness autosomal recessive 9, DFNB9). Several studies have demonstrated expression of otoferlin in the inner ear and brain, and suggested a role of otoferlin in Ca(2+)-triggered exocytosis. So far, otoferlin expression profiles were solely based on the detection of mRNA. Here, we analysed the expression of otoferlin protein and mRNA using immunohistochemistry, in situ hybridization and RT-PCR in neonatal and mature Wistar rat tissue. In agreement with previous studies, otoferlin expression was found in the brain and in inner and vestibular hair cells. Otoferlin mRNA and protein was, however, also detected in mature outer hair cells of low-frequency processing cochlear turns and in auditory nerve fibres. In outer, inner and vestibular hair cells, otoferlin was subcellularly localized at a considerable distance from the presumed active release sites. Double-staining with the synaptic ribbon marker, C-terminal binding protein 2 (CtBP2), or the presynaptic Ca(2+)-channel, Ca(v)1.3, both assumed to mark the sites of vesicle fusion and transmitter release, did not colocalize with otoferlin expression and thus do not necessarily support a selected role of otoferlin in Ca(2+)-triggered exocytosis. The widespread distribution of otoferlin in neurons, nerve fibres and hair cells, and its subcellular distribution extending beyond the regions of synaptic vesicle fusion, i.e. coenrichment with the cytosolic Golgi matrix protein 130 (GM130) in inner hair cells or the early endosomal autoantigen 1 (EEA1) in outer hair cells support instead the idea of a more ubiquitous role of otoferlin in early/recycling endosome trans-Golgi network dynamics.

Sphingosine 1-phosphate (S1P) signaling is required for maintenance of hair cells mainly via activation of S1P2

Hearing requires the transduction of vibrational forces by specialized epithelial cells in the cochlea known as hair cells. The human ear contains a finite number of terminally differentiated hair cells that, once lost by noise-induced damage or toxic insult, can never be regenerated. We report here that sphingosine 1-phosphate (S1P) signaling, mainly via activation of its cognate receptor S1P2, is required for the maintenance of vestibular and cochlear hair cells in vivo. Two S1P receptors, S1P2 and S1P3, were found to be expressed in the cochlea by reverse transcription-PCR and in situ hybridization. Mice that are null for both these receptors uniformly display progressive cochlear and vestibular defects with hair cell loss, resulting in complete deafness by 4 weeks of age and, with complete penetrance, balance defects of increasing severity. This study reveals the previously unknown role of S1P signaling in the maintenance of cochlear and vestibular integrity and suggests a means for therapeutic intervention in degenerative hearing loss.